1. Field of the Invention
The present invention relates to an image sensor and an image processing apparatus adopted in camcorders, digital cameras, etc. More particularly, the invention relates to an image sensor for semiconductor light-sensing device which uses a color filter array having open windows and single-color filters alternating with each other, using information of a plurality of colors for each pixel to allow more accurate interpolation, thereby extracting a more accurate color for each pixel, and to an image processing apparatus using the same.
2. Description of the Related Art
In general, the recent developments in the picture communication by wire/wireless high-speed network system, and the picture input and recognition technology such as used in digital cameras have led to increased use of digital camera modules in mobile telecommunication terminals such as mobile phones. Accordingly, there have been active research and development in image sensors adopted in the digital camera modules.
Such an image sensor is a sensor which senses light reflected by an object to sense an object image. It is broadly divided into a Charge Coupled Device (CCD) type and a Complementary Metal Oxide Semiconductor (CMOS) type depending on the manufacturing technology.
The CCD image sensor moves an electron generated by light to an output part using a gate pulse. In this process, the voltage may change due to external noise but no change is incurred in the number of electrons. Thus, the noise does not affect the output signal, which indicates superior characteristics against noise. Because of such a merit, the CCD type image sensor is extensively used in multimedia devices such as digital cameras and camcorders which require a superior picture quality.
On the other hand, the CMOS image sensor (CIS) converts an electron generated by light into a voltage in each pixel and output the voltage afterwards through multiple CMOS switches. At this point, the voltage signal may be degenerated due to noise, which indicates mediocre characteristics against noise. However, as the merits of the CIS such as low manufacturing costs, low power consumption and possible integration with nearby circuits, compared with the CCD type, became widely known, there have been efforts to improve the CMOS processing technology as well as signal-processing algorithm in the late 1990s. Thus, the existing shortcomings have been improved over time, and recently there have been more active researches on the CIS.
The most representative technology related to the image sensors includes the Bayer pattern of a color filter array, and three-photodiode color sensor which allows sensing all three colors in each pixel. The Bayer pattern technology is used in most of the image sensors today, in which the three colors are separated by a color filter so that light is sensed at each pixel.
FIG. 1 is a block diagram illustrating a conventional image processing apparatus.
With reference to FIG. 1, the conventional image processing apparatus is composed of an image sensor 10 including a plurality of microlenses 11, a color filter array 12, a protection layer 13, and a pixel sensor array 14, and a signal processor 20 for processing signals such as via interpolation of color signals R, G, B from the image sensor.
Here, the color filter array 12 adopts the Bayer pattern, illustrated in FIG. 1, which is used in most of the image sensors including the CCD type.
FIG. 2 is a diagram illustrating the Bayer pattern of the color filter array shown in FIG. 1.
Referring to FIGS. 1 and 2, the color filter array 12 adopts the widely known Bayer pattern. The Bayer pattern is composed of basic units each made up of 2 by 2 cells, in which two green filter are disposed diagonally and one of each red and blue filters are disposed diagonally. Since the human eye is more sensitive to green than red and blue, the green filter area is composed of two cells.
The pixel sensor array 14 includes a photosensor for receiving light by a photodiode, and a signal detector for outputting a signal generated by the photosensor. The miniaturization of the photosensor and the signal detector is the core technology of the CMOS image sensor. The photosensor is a photodiode of a general P-N junction structure which is compatible with the general CMOS image sensor, thus widely used in CMOS image sensors. The structure of the pixel sensor array including such a photosensor is as shown in FIG. 3.
FIG. 3 is a block diagram illustrating the pixel sensor array of the image sensor shown in FIG. 1.
Referring to FIGS. 1 and 3, the pixel sensor array 14 includes a P+ substrate 14-1, a P-epitaxial layer 14-2 grown on the P+ substrate 14-1, an n-well layer 14-3 formed on the P-epitaxial layer 14-2, which forms a photosensor of a single P-N junction structure capable of receiving light, and a P+ shallow junction layer 14-4 which is formed of P+ semiconductor material in a predetermined depth from an upper surface of the P-epitaxial layer 14-2. The N-well layer has a junction depth of about 0.6 μm.
In addition, the signal detector converts a signal, which is converted from light by the photosensor, into a voltage to be outputted to the signal processor 20.
FIGS. 4(a) and (b) are diagrams for explaining interpolation by the signal processor shown in FIG. 1.
Referring to FIGS. 4(a) and (b), the interpolation process by the signal processor 20 is determined by the pattern of the color filter adopted in the image sensor 10.
With reference to FIG. 4 (a), in the case where a unit for interpolation is set as 2 by 2 cells, the 2 by 2 interpolation is conducted according to following Equation 1, thereby obtaining the color information ri, gi and bi for red R, green G and blue B with respect to an arbitrary pixel.ri=Rgi=(G+G)/2bi=B  Equation 1
Here, ri, gi and bi are RGB color information for an arbitrary pixel, obtained from the interpolation.
As shown in Equation 1, the RGB color information of each pixel is obtained from the interpolation. Here, the green information gi is obtained on the basis of two color signals in an arbitrary pixel but the red information ri and the blue information bi are obtained on the basis of only one color signal respectively. Thus, if the red R and blue B signals are distorted by noise in this case, this may be a cause for an error in the CMOS image sensor such as the fixed pattern noise. That is, if white and dark noises take up large portions in red and blue signals due to lack of uniformity in the manufacturing process, the interpolation becomes less accurate due to the fixed pattern noise. This is more severe in the case of highly dense pixel arrangement with small pixels.
In order to overcome such a shortcoming, 3 by 3 interpolation is sometimes used in high-density image sensors, which will be explained below with reference to FIG. 4 (b).
Referring to FIG. 4 (b), with the Bayer pattern of the color filter array 12, 3×3 interpolation is conducted according to the following Equation 2, thereby obtaining red, green and blue color information ri, gi and bi.
1. 3×3 interpolation in red pixelri=Rgi=(G+G+G+G)/4bi=(B+B+B+B)/42. 3×3 interpolation in green pixelri=(R+R)/2gi=(G+G+G+G+G)/5bi=(B+B)/23. 3×3 interpolation in blue pixelri=(R+R+R+R)/4gi=(G+G+G+G)/4bi=B  Equation 2
As shown in Equation 2, in the red pixel, the green G information and blue B information gi and bi is obtained from an average value of the adjacent 4 color signals but the red R information ri is obtained from a single color signal. In the blue pixel, the red R and green G information ri and gi is obtained from an average value of the adjacent 4 color signals but the blue B information bi is obtained from a single color signal. As such, 3 by 3 interpolation allows obtaining-color information from an average value of maximum 5 color signals, and thus not affected as much by the fixed pattern noise as 2 by 2 interpolation. Still, 3 by 3 interpolation includes a case of obtaining color information from a single color signal, sharing the problem exhibited in 2×2 interpolation.
As described above, in the conventional image sensor and the image processing apparatus, the color information for each pixel is obtained on the basis of a single color signal. In that case, if red R and blue B signals are distorted by noise, there occurs an error such as the fixed pattern noise in the CMOS image sensor. Also, if white and dark noises take up large portions due to lack of uniformity in the manufacturing process, the interpolation becomes less accurate due to the fixed pattern noise, which does not allow extraction of accurate color information for each pixel.